The present invention relates to photo semiconductor devices having a structure which enables changes in refractive index of a desired portion and, more specifically, to photo semiconductor devices such as an optical modulator and an optical switch. More particularly, the present invention pertains to a photo semiconductor device which may suitably be applied to an optical integration element, optical switchboard, or the like.
It is described in detail in Japanese Patent Laid-Open No. 134219/1985 and its corresponding European Patent Laid-Open No. 0147195 that a change in refractive index which results from injection of carriers into a semiconductor material is larger than a refractive index change caused by the electro-optic effect or the like and threrefore applicable to optical switches or the like.
Since the refractive index change by carrier injection is estimated to be greater in the case of the multi-quantum well (MQW) structure than that in the case of the bulky crystal structure, a combination of these techniques theoretically enables attainment of a photo semiconductor device which is operable at high efficiency, for example, a low threshold semiconductor laser, high-efficiency optical modulator or optical switch.
In the above-described prior art, carriers are injected into thin semiconductor layers which constitute the MQW structure in a direction perpendicular to the surface of the semiconductor layers. The MQW structure is formed by alternately laminating a well layer and a barrier layer. Therefore, when carriers are injected perpendicularly to the surface of the MQW structure, the injected carriers move while being obstructed by the barrier layers and successively fill the well layers from those which are closer to the carrier injection side. Substantially no carriers reach the well layers which are 0.1 to 0.2 .mu.m or more away from the carrier injection side. Accordingly, when there is a need for an optical waveguide (including all the MQW layers) having a relatively large thickness, e.g., 5 .mu.m or more, in the case, for example, of a large-output laser, optical modulator or optical switch, a considerably small amount of carriers is injected into the well layers which are 0.1 to 0.2 .mu.m or more away from the carrier injection side, so that effects caused by carrier injection, for example, changes in the refractive index, are unsatisfactory in these well layers. Therefore, it is impossible to expect satisfactory improvements in the characteristics of a photo semiconductor device, for example, extinction ratio.